1. Field of the Invention
The present invention relates to navigation systems, and more particularly, to a vehicle navigation apparatus adapted to effectively obtain navigation information in a navigation system which comprises a vehicle-mounted terminal (navigation apparatus) and a server connected in communication with the terminal for guiding a vehicle along a guidance route from a present position of the vehicle to a destination based on a requirement from the terminal, and to a method for obtaining navigation information using the same.
2. Description of the Related Art
Conventional navigation apparatus are provided with a CPU for controlling all procedures associated with navigation, a storage device, such as a CD-ROM or a DVD-ROM, for storing therein map data, a display device, a global positioning system (GPS) receiver for detecting a present position of the vehicle, a gyro and/or vehicle speed sensor for detecting the direction of travel and/or the traveling speed of the vehicle, and a vehicle information and communication system (VICS) receiver for receiving traffic information. With the CPU, map data including information about the present vehicle position is read from the storage device. Based on the map data, a map image covering the vehicle position and its surroundings is displayed on a display screen with a vehicle position mark indicative of the present vehicle position superimposed on the screen. As the vehicle travels, the map image is scrolled with the vehicle position mark fixed on the screen, or otherwise the vehicle position mark is moved on the image with the map image fixed on the screen, thus enabling a user to understand the present traveling position of the vehicle at first glance.
A vehicle navigation apparatus normally has a function of route guidance (route guidance function) that enables a user to easily travel an appropriate route to a desired destination without taking a wrong path. This route guidance function searches for the appropriate route with the CPU from a starting point (which is typically a present position of the vehicle) to the destination using the map data by simulation calculation, such as a lateral search method or a Dijkstra method. The searched route is stored as a guidance route, and then displayed on the map image in a manner distinguished from other roads (e.g. in a different color, or in a different line thickness) during the vehicle's traveling. Alternatively or additionally, when the vehicle is approaching a point located at a predetermined distance before an intersection where a next turn is to be made on the guidance route, a guidance map concerning the intersection (an enlarged view of the intersection, an arrow indicative of a traveling direction at the intersection, a distance to the intersection, a name of the intersection, or the like) is displayed on the map image, so that the user can understand on which road the vehicle should travel, or in which direction the vehicle should progress from the intersection. Such a navigation apparatus capable of performing route search using only the map data installed on the vehicle (i.e., “on board”) is called “local navigation”.
On the other hand, “communication navigation” proposed as another navigation type has recently attracted increased attention. This navigation type is designed to perform route guidance based on the result of route search obtained from outside of the vehicle by communications (i.e., “off board”). The typical communication navigation system sends positional data concerning the present vehicle position and the destination to a server (information center), which is connected to the vehicle through the Internet or the like, from the user's vehicle through a portable telephone. Based on the positional data, an appropriate guidance route (recommended route) to the vehicle is searched for or retrieved on the server side, and the result of searching is downloaded by communications, thereby carrying out route guidance of the vehicle.
In the case of communication navigation, the amount of data in a database installed on the center side is large, and the contents of the data are generally updated whenever necessary. The latest updated map data (road information, point of interest (POI) information, VICS information, or the like) is consistently kept in the database. Thus, this type of navigation has an advantage that the most accurate guidance information can be provided to the vehicle-mounted terminal (navigation apparatus). On the other hand, such a navigation system has the following disadvantages. On the vehicle-mounted terminal side, information needed for route guidance of the vehicle must be obtained from the center by communications, resulting in high communication costs. Additionally, the route guidance is carried out while downloading the necessary information. This communication requires much time, which leads to poor responsiveness of the navigation.
In such a known communication navigation apparatus, a measure has been developed to decrease the number of possible communications with the center so as to reduce the time and cost required for the communications. As an example, a new type of navigation apparatus has been proposed which is designed to obtain data about routes in starting route guidance, and then to carry out the route guidance after obtaining all route data.
As one approach associated with the above-mentioned “communication navigation”, a communication navigation system is disclosed in JP-A-11-339182, for example, which can obtain the most appropriate route information according to the current condition of the vehicle, while achieving cost savings in communications and avoiding new complicated communication techniques.
The conventional “communication navigation” type of navigation apparatus is adapted to minimize the amount of information included in the map data (data about routes, data including a map associated with a surrounding area of the route, or the like) obtained from the center so as to decrease communications traffic. If the vehicle has deviated from the guidance route (that is, in the case of off-route), the apparatus must communicate with the center again to obtain navigation information needed for the vehicle to return to the guidance route (hereinafter referred to as “map data for rerouting”).
In this case, however, it is necessary to take some time to access the center so as to establish communications (communication connection time), as well as some additional time to obtain the map data for rerouting from the center based on the access (data obtaining time). As a result, the operation speed of the apparatus becomes low (that is, the responsiveness thereof is reduced), which proves problematic from a practical viewpoint. That is, since the conventional communication navigation apparatus is designed to access the center when the vehicle goes off-route, the map data for rerouting (navigation information) disadvantageously cannot be obtained until the total time of at least the communication connection time and the data obtaining time has passed since the time of going off-route.
To deal with such inconvenience, some communication navigation apparatus are provided for previously obtaining map data with a predetermined map width. For example, the navigation is designed to previously obtain information on roads and POI located within a range of 300 m on each side of the guidance route, or the like. Since this communication navigation apparatus must obtain a relatively large amount of data, the communications traffic, in other words, communication time is increased, which disadvantageously results in high communication costs.